Luis von Ahn (co-inventor of reCAPTCHA), в личной беседе с автором,15 апреля 2014 года.
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Google. (2014). Digitalizing books one word at a time. По данным сайтаhttp://www.google.com/recaptcha/learnmore.
von Ahn, L., Maurer, B., Mc Millen, C., Abraham, D., & Blum, M. (2008). reCAPTCHA: Human-based character recognition via Web security measures. Science, 321(5895), 1465–1468. reCAPTCHAs act as sentries Google. (2014). Digitalizing books one word at a time.
